So いい子でいる is more like "to be (in the state of being) a good child", and therefore more instantaneous/specific than the more general いい子である, right? In other words, いい子でいる would only ever point to an instance of a child being good (well-behaved, etc.), whereas いい子である would be more about the general characteristics of that particular child being good (well-behaved, etc.) in general, or under usual circumstances. In other other words (I just kind of made that up), いい子でいる = "a good, well-behaved child in this instance/right now", vs. いい子である = "a good child most of the time (anyone would think this child is a good child no matter how they're acting right now)". I hope that's not too convoluted. At any rate (assuming it isn't), is that a fair assessment of the difference between the two?